1. Field of the Invention
The invention relates to an improved process for preparing dimethyl carbonate (DMC) by oxycarbonylation of methanol in the presence of Cu salts.
Dimethyl carbonate is an intermediate having low toxicity and can replace toxic intermediates, such as phosgene or dimethyl sulphate, in many reactions. Furthermore, it is not corrosive. Its use gives no environmentally damaging by-products.
Examples of such reactions of dimethyl carbonate are the preparation of urethanes from aliphatic or aromatic amines, which in turn can be cleaved to give the corresponding isocyanates. Dimethyl carbonate can, for example, also replace dimethyl sulphate in the quaternization of amines or in the methylation of phenol or of naphthols. Dimethyl carbonate can also be added to motor vehicle fuel to improve the octane number, e.g. in place of lead compounds. This importance of dimethyl carbonate has prompted the search for a technically simple and environmentally friendly production process which is suitable for large capacities without significant by-product formation or coupled material circuits.
2. Description of the Related Art
To prepare dimethyl carbonate, there are various preparative processes which have been tested on a small scale and also industrially. The preparative routes which are based on the catalytic reaction of methanol with carbon monoxide and oxygen in accordance with the following equation have been intensively studied by various groups of workers: ##STR1##
The copper compounds acting as catalysts have thus been used in the form of various copper salts. Use of copper(II) chloride as catalyst in accordance with JP-45/11129 (1970) gives unsatisfactory selectivities. Particular problems are caused by the formation of relatively large amounts of methyl chloride which, because of its high volatility, tends to distribute itself ubiquitously throughout the entire production plant and can lead to corrosion in virtually the whole plant.
Better selectivities are obtained by using organic complexing agents (DE-A 21 10 194), but this gives the problem of separating off the catalyst salts which are partially dissolved in the reaction mixture, but the major part of which is present as a suspension.
Particularly problematical is carrying out this reaction in accordance with DE-A 27 43 690, since the catalyst salts in the reaction mixture are virtually completely undissolved, but are merely suspended. These salts have to be conveyed through the reaction zone and through the cooling equipment and after the reaction have to be separated off mechanically, e.g. by means of centrifuges. Besides the corrosion already mentioned, this causes erosion, poor heat transfer and also blockages and encrustations.
To avoid these disadvantages of a catalyst circuit, it has been proposed that the catalyst salts be retained in the reactor in suspended form and that methanol, CO and oxygen be metered into the reactor, while the dialkyl carbonate formed and the water of reaction are distilled out of the reactor together with methanol used in excess (EP 0 413 215 A2). The liquid reaction medium here consists essentially of the methanol to be reacted (EP-0 413 215, page 3, line 52), so that the molar ratio of methanol to Cu salt is very high (preferably 1:0.01-0.05). This has the disadvantage that the reaction rate is relatively low. A problem here is also the need to set a low dimethyl carbonate concentration. This is not easy, since the reaction is carried out at a high system pressure and the solubilities of dimethyl carbonate and also of water in the reaction medium, which consists essentially of methanol, are very high. This means that the separating off of dimethyl carbonate and water has to be forced by means of a relatively large amount of inert gas or methanol gas.
In a relatively new process in accordance with German Offenlegungsschrift 41 38 755, it has been shown that the reaction rate for the oxycarbonylation of methanol can be substantially increased if Cu compounds in the form of molten salts at from 120.degree. to 300.degree. C. are used.
The particular advantage of this process becomes valuable when the water content in the reaction mixture can be kept low (Patent Application P 43 25 651.1 of 30.7.1993).